ABSTRACT The purpose of the study is to investigate if a monetary incentive can increase the response rate for vulnerable children and youth in a postal survey. A secondary purpose is to test if monetary incentive has an effect on the content of answers in a postal survey. The study is a randomized controlled trial where participants will be randomly allocated to receive a questionnaire together with a €15 voucher for a supermarket or to only receive a questionnaire. Data are taken from an intervention study aiming at creating network for different groups of vulnerable children and youth aged 8-23 years. The questionnaire survey in the present study is a follow up survey of the original study which are performed 1-2½ years after the participant have left the study. Recruitment to the original study is still ongoing and the investigators expect the total participation to be around 250. In the primary analysis the investigators will estimate the relative risk (RR) to assess whether there is a difference between group allocation and questionnaire response rate. The investigators will adjust for age in the analysis. In the secondary analyses the investigators will test whether scores on the five SDQ scales are different between group allocations. The investigators will adjust for the covariates age and gender, since the scores may depend on age and gender (4).